reaction.added fires when a contact adds a reaction to one part of a message.
Read the payload
Thedata object is the ReactionEventBase schema from the API contract. The envelope around it is the same on every event and is described in the event catalog.
Handle the event
Attach the reaction to partpart_index of message message_id in your stored copy. is_from_me is true when your own agent reacted, which you can ignore. A custom reaction carries its emoji in custom_emoji; the six standard tapbacks carry null there.

